Servocontrol system for discharge sintering



April 21, 1970 KIYOSHI lNOUE [3,508,029 SERVOCONTROL SYSTEM FOR DISCHARGE SINTERING Filed July 11. 1967 4 Sheets-Sheet 1 P0 TEN TIAL INVENTOR. lr/ YOSH/ INOUE A TUR E X April 21, 1970 KIYQSHI INOUE Q 3,508,029

S'ERVOCONTROL SYSTEM FOR DISCHARGE SINTERING Filed July 11, 1967 4 sheet -sheet z k/yasu/ y /$22" F IG. 2

CKar CR0 April 21 19 70 KIYOSHI INOUE 3,508,029

- SERVOCONTROL SYSTEM FOR DISCHARGE SINTERING I Filed July 11. 1967 4 Sheets-Sheet 5 April 21, 1970 KIYOSHI INOUE SERVOCONTROL SYSTEM FOR DISCHARGE SINTERING I Filed July 11, 1967 4 Sheets-Sheet 4.

INVENTOR. K/YdS/l/ I/VOl/' arl jams 4 TIOENE/ U.S. Cl. 219-149 14 Claims ABSTRACT OF THE DISCLOSURE A servocontrol system for a movable electrode of a discharge system, especially for the spark-discharge sintering of conductive and nonconductive particles, Wherein reversible drive means is provided for the movable electrode, and a servomechanism responsive to a parameter of the conditions of the gap between the electrodes (generally containing the sintered particles or the presintered mass of discrete particles), detects the parameter and converts it into a control signal for operating the drive means whereby the movable electrode follows the shrinkage of the sintered mass. Programming means (e.g. a cam) can be connected in the servocontrol circuit for augmenting the response of the drive means to the control signal when, for instance, it is desired to increase the pressure applied by the moving electrode to the mass of particles or the speed thereof after an initial movement at a lesser rate.

SPECIFICATION This application is a continuation-in-part of my copending application Ser. No. 356,714 filed Apr. 2, 1964 (now U.S. Patent No. 3,340,052) as a continuation-inpart of application Ser. No. 247,387 of Dec. 26, 1962, now U.S. Patent 3,250,892 dated May 10, 1966, and a continuation-in-part of my copending application Ser. No. 340,211, Jan. 27, 1964, now U.S. Patent No. 3,364,333 dated Jan. 16, 1968.

My present invention relates to servocontrol system for the electrical sintering of discrete elements into a coherent body and, more particularly, to systems for controlledly advancing a movable electrode of a spark-sintering system to maintain predetermined discharge conditions and pressures even during shrinkage of the mass upon partial coalescence of the particles.

In the above-mentioned applications, which relate to various developments in the method of spark discharge sintering, it is pointed out that spark sintering of discrete elements (e.g. conductive particles or mixtures of conductive and nonconductive particles) are able to achieve results not possible 'by priorconventional sintering methods. In these earlier methods, a mass of particles is heated, usually in electric or gas furnaces and under an antioxidizing atmosphere, to temperatures sufiicient to cause partial coalescence of the intercontacting particles and with extremely high pressures, generally on the order of tons/cm In effect, therefore, the process is similar to ordinary spot Welding techniques where, under sufficient temperature and .pressure, portions of the elements to be joined flow together to a. certain extent and bond one with the other. While various sintering arrangements have'been suggested utilizing these. techniques, resistance heating and inductive heating of the bodies appear to predominate.

In my above-identified Patent No. 3,250,892 and the copending application set forth above, as well as my co- United States Patent "ice pending application Ser. No. 611,497, filed Nov. 30, 1966 as a division of application Ser. No. 356,714, it is pointed out that one of the principal barriers to mutual bonding of metallic particles, for example, is the presence of microscopic or submicroscopic films or skins upon these particles which impede mutual intercrystalline dilfusion of the particulate materials. Thus it has been hypothesized that the high temperatures required for prior sintering techniques and the reducing atmospheres commonly employed therefor, at least partly destroy this film and permit the elevated static pressures to bond the particles together. In the spark-discharge technique, however, it appears that a spark discharge is generated among the particles, at least predominantly conductive, between the electrodes in contact with the particulate mass and in such light-contacting relationship that a discharge is generated across the electrodes and through this mass when an electric-current pulse is applied across the electrode. This discharge appears to pass through the mass from one electrode to the other and to be a composite of numerous discharges between the individual conductive elements and the nearby elements of the mass. It may be assumed that the discharge initially erodes or destroys impulsively any oxide coating which may be present on the particles and also urges the particles together with a dynamic pressure or shock wave. Furthermore, as noted in the aforementioned applications, material is carried by the individual discharges from one particle to another to form, in effect, conductive bridges between the particles. Further current flow is concentrated at these bridges so that the latter are heated practically to a state of coalescence and the bridged particles grow together into a practically monocrystalline structure.

It has already 'been inferred that the particle mass may be composed at least in part (erg. up to 20% by volume) of nonconductive particles which preferably are thermally fusible to the conductive particles or are trapped in the interstices of the coherent mass. The nonconductive particles may be particles of a metal oxide or of a synthetic resin, preferably a thermoplastic, whereas the conductive particles are usually particles of metal but may partly be metallic particles and partly particles of graphite. The term particles as used herein is intended to include elements or bodies of any configuration, e.g. metallic or nonmetallic whiskers, fibers, filaments, polyhedrons, spheres, and the like.

As also set forth in these applications, the develop ment of the sintering discharge can be facilitated by modifying the electric potential across the mass .at a relatively high frequency by vibrating one or both of the electrodes at sonic or ultrasonic frequencies and/or by applying a high-frequency periodic or alternating current across the electrode and in superimposition upon the discharge current.

It has been found that it is critical to control the position of the movable electrode as well as the pressure which is applied to the mass continuously during the course of the sintering operation in spite of the fact that an initial fusion occurs substantially instantly inasmuch as the subsequent sintering stages involve the propagation of spark discharge through the mass. Consequently, while it is possible to permit the movable electrode to follow the shrinkage of the mass under its own weight or with a light resilient-bias, this technique does not suffice for accurate control of the sintering step and often leads to nonreproducible products. 7

It is, therefore, the principal object of the present invention to provide an improved method for the electrical sintering of discrete elements into coherent bodies and especially for the spark-discharge fusion of such elements which extends the principles originally set forth in the aforementioned copending applications and my earlier patents in this field.

A more specific object of this invention is to provide a method of controlling the spark-discharge sintering of discrete elements into coherent bodith whereby these bodies are reproducible and of predictable quality. Yet another object of my invention is to provide a servocontrol system for a spark-discharge sintering apparatus, which is capable of improving both the rate of sintering and the quality of the sintered product.

According to a principal feature of the present invention, a spark-discharge sintering apparatus includes at least one movable electrode adapted to retain a mass of electrically sinterable particles between itself and a counterelectrode, a drive or motive means for controlledly advancing the movable electrode toward the counterelec- .trode and thereby decreasing the volume of the chamber containing the sinterable particles, and a servocontrol means responsive to at least one electrical parameter of the sintering system connected across the mass and coupled with the drive or motive means for operating the latter in accordance with the sensed parameter to advance the movable electrode in step with or at a rate determined by the shrinkage of this mass.

Thus, the present method includes the step of ascertaining or detecting variations in a parameter of the sintering system across the electrodes and using this parameter to control positively the advance of the electrode. According to a specific feature of this invention, means are provided in the servocontrol system for varying the rate of advance of the electrode with change in the sensed parameter for producing special effects in the finished body.

For example, in application Ser. No. 326,837, now issued 'as US. Patent No. 3,317,705, I point out the advantage of increasing the pressure applied to the partially fused mass at least after the initial formation of fusion bridges by spark discharge. Thus, in the system of the present invention, the servocontrol system can include a mechanism for increasing the rate of advance of the movable electrode and thus the pressure applied thereby to the sintered mass at a subsequent stage in the sintering operation.

The motive means of this invention can include a fluidoperated cylinder while the servocontrol means can include pneumatic, hydropneumatic or hydraulic valve means for supplying a fluid cylinder or rotary fluid motor. The valve means may be advantageously operated electrically either by an electrical/fiuidics control system as described in my copending application Ser. No. 608,476,

-filed Dec. 19, 1966 or by electromechanical valve members such as control reeds. Alternatively, a reversible DC. motor may be used as the motive source.

According to a further feature of this invention, the electrical parameter of this system is sensed by a Halleffect crystal having at least two electrical inputs one of which is proportional to the current flow through the sintered mass while the other is proportional to the voltage across the electrodes. Thus, the servocontrol system includes a device for forming a product of current and voltage to produce a control signal representing the power applied to the mass. I have found that regulation of the advance of the electrode by at least two electrical parameters of this system, namely, the voltage across the gap and the current flow between the electrodes or, when possible, the power consumed in the sintering operation directly, is highly advantageous.

It has been found, surprisingly, that a purely D.C. source (and with somewhat lower efficiency a low-frequency A.C. source) can be effectively employed for spark sintering when the servosystems are used to control the advance of the electrodes, in spite of the fact that DC. power has hitherto degenerated into a continuous current flow substantially instantaneously. It is however advantageous to promote the formation of fusion zones between the particles by the application of current surges or pulses superimposed on the DC. component which induces electrotransportation of material within the sinterable mass. When the load current is periodic or changeable in the course of sintering, a dispersion of the sparks is produced, with a more widespread distribution of the sinter points along the particle surfaces.

According to another aspect of this invention, the sparksintering power source includes, in addition to a DC. source supplying the unidirectional component, a solidstate pulse source having a thyratron type switching element (i.e. a solid-state controlled rectifier) in series with a discharge capacitor and the discharge assembly. A quenching choke can be provided in the anode circuit to terminate current flow upon the development of inherent oscillation, while the gate circuit preferably includes a unijunction-transistor oscillator. The bias of the emitter/ base network of the latter may be varied to change the frequency pulse width and interval in a relatively simple manner and in accordance with the material to be sintered.

Thus the power supply for spark sintering (with or without use of the powder layer to weld two solid bodies together or to fuse the sinterable mass to one solid body) may include a source of direct current having, say, 3% ripple, or a pulsating-current source of low or high frequency derived from a full-wave or half-wave rectifier. Other suitable power supplies, some of which are described in greater detail herein-below, may include capacitor-discharge arrangements, systems using capacitive discharge or pulsating sources as generators of the unidirectional component upon which low or high frequency components may superimposed, electronic switch, oscillator or multivibrator arrangements producing rectangular or-sawtooth or equivalent waveforms, and combined circuitry whereby the pulse train is superimposed upon D.C. Furthermore, the control of the electrodes can be advantageously carried out in various modes. For example, the'upper electrode alone may respond to the servocontrol signal and may thus follow the gap parameters durlng the initial stage of sintering and may then apply a high terminal pressure (cf. the aforementioned copending applications as to the significance of the elevated terrnlnal pressure). Both the upper and the lower electrodes may be initially servocontrolled and thereafter operable to apply the high terminal pressure, or the upper electrode may be designed to apply the elevated terminal pressure whereas the lower electrode follows the shrinkage of the sintering mass by detection of the electrical parameters of the gap. It has been found that this latter arrangement provides the ability to promote spark discharge, especially when combined with vibration of the lower electrode at sonic or ultrasonic frequencies, in view of the critical cooperation of gravitational compaction of the mass and elect-rode movement.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a diagrammatic cross-sectional view showing a spark-sintering apparatus according to this invention;

FIG. 2 is a view generally similar to FIG. 1 of a somewhat more complex system using a fluid-responsive servomechanism adapted to respond to several parameters of the sintering step;

FIG. 3 is a diagrammatic perspective view of another servocontrol system in accordance with the present invention;

FIG. 4 is a graph illustrating various aspects of the invention;

FIG. 5 is a diagrammatic view of still another sparksintering system according to this invention;

FIG. 6 is a perspective view, in diagrammatic form, illustrating a welding arrangement using principles of spark sintering; and

FIG. 7 is a diagram of still another spark-sintering system applying the invention.

The apparatus of FIG. 1 can be used in accordance with the present invention for the production of sintered bodies under continuously controlled conditions. In this system, the nonconductive :mold 1 is disposed upon an electrode plate 2 and contains the electrically sinterable power 3 which islightly compressed by a movable electrode 4. The mechanical pressure of the latter is determined by the position of its piston 5 which is received in a hydraulic cylinder 6. Hydraulic fluid is supplied to the cylinder by a distributing valve 7 which is fed by a pump 8 from a reservoir 9. The distributing valve 7 is controlled by a solenoid coil 7a, forming a mechanism responsive to the voltage drop across the electrodes 2 and 4. Thus, the solenoid coil 7a is bridged across the electrodes 2 and 4 and is responsive to the potential drop through the sintered-particle mass 3. It will be immediately apparent that this potential drop is a direct measure of the density of the body formed by the particles since greater compaction involves a decreased resistance thereacross. In this system, the motive or drive means of the servomechanism is a fluid-responsive device as represented by the piston-and-cylinder arrangement 5, 6 and the solenoid coil 7a is responsive only to potential drop. When a constant-voltage source is used the potential drop is proportional to the current flow and resistance.

Additionally, I provide a magnetostrictive vibrator for oscillating the electrode 4 in the direction of compaction of the particles within predetermined limits. The vibrator comprises a pair of windings 4a and 4b connected in adding relationship and wound about the shanks of a D-shaped coil 40 of magnetostrictive material. These coils are energized by an alternating-current source 4d whose output is superimposed upon a direct-current biasing voltage derived from battery 4e. A capacitor 4g is tied in series with source 4d across the coils 4a, 4b as a DC. blocking impedance, while a surge-suppressing choke 4] is serially connected with battery 4e.

The direct-current source for the discharge electrode 2, 4 is constituted by a rectifierbridge 2a fed by one secondary winding of a transformer 2b whose other secondary superimposes an alternating current upon the elec-' trodes via a DC. blocking capacitor 20. The transformer 2b is energized by the alternating current source 20. while the discharge capacitor 22 is connected across the electrodes 2, 4.

In practice, the initial discharge across the electrodes 2, 4 causes the formation of fusion bridges between the'particles and a decreased voltage drop across the particle mass. The solenoid 7a senses this drop and advances the electrode 4 at a rate permitting further discharge without application of substantial pressures so that the impulsive forces associated with spark discharge and the oxidestripping eifects thereofv continue. The electrode 4 thus follows the shrinkage of the mass 3.

In FIG. 2, I show a system in which a movable electrode 10 is shiftable in the direction of the fixed electrode plate 11 by a hydraulic-cylinder assembly 12 to compact a mass 14 of particles to be sintered. The mass 14 is held in place by a sintered mass of other particles 15 retained by a band 16. The fluid-responsive electrodedrive assembly is mounted upon a stand 60 Whose base 61 carries theelectrode plate 11.

The spark-sintering power supply 17 for the apparatus can include a spark-discharge capacitor 18 connect d across the electrodes 10 and 11in series with a surgesuppressing inductance 19; the capacitor 18 is charged by a battery 20 through the charging resistance of the circuit.

As can be seen from FIG. 2, the power supply 17 includes a pulsing system represented by the battery 20, the surge-suppression choke 19, the capacitor 18 and a silicon solid-state controller rectifier 17 in series with the capacitor 18 and the electrodes 10, 11. The gate of the controlled rectifier 17 which is quenched upon the decay of the forward bias in the anode-cathode circuit or by a reversal thereof, is triggered by an oscillator represented at 172 as illustrated in greater detail at 317a and described below in connection with FIG. 5. This oscillator is an electronic switching device adjustable with respect to pulse width, frequency and spacing and may be constituted as a standard unijunction-transistor oscillator transformer coupled to the gate of the controlled rectifier. The anode-cathode circuit of the controlled rectifier includes a quenching choke 17d across which the electrodes 10 and 11 are connected and adapted to produce oscillations for reverse biasing of the control rectifier to quench it after each firing. In addition to the pulsating source 17, 17c, the power supply of the system of FIG. 2 includes a unidirectional source represented as a battery 17g in series with the thyratron-type switch 17) and a discharge capacitor 18 in the electrode-supply circuit. A pulsepassing capacitor 17i is connected in shunt with the battery 17g so as to pass the pulses arising from the triggering of the control rectifier 17 and to superimpose such pulses upon the unidirectional field.

The servocontrol system of this invention, which may be similar to those used in electric-discharge machining, can comprise a pneumatic, hydropneumatic or hydraulic servocontrol valve assembly 62 (as generally described in Servomechanism Practice, Ahrendt & Savant, McGraw- Hill Publishing Company, New York, 1960; pp. 402- 422) connected with a fluid source 63 which, in turn, is supplied from a reservoir 64 to which excess fluid is returned. The assembly 62 comprises a pair of nozzles 65a and 65b connected respectively to the opposite sides 66a and 66b of a control valve 67 whose valve member 68 is reciprocable to control the hydraulic fluid flow through a low-pressure return line 69a and a high-pressure supply line 69b of the fluid system. The chambers 66a and 66b are, in turn, supplied with fluid under pressure from a duct 70 so that, as a reed 71 blocks or unblocks the nozzles 65a and 65b, pressure is built up or relieved in the respective chamber 66a and 66b and the valve member 68 biased accordingly. When, for example,

the nozzle 65b is blocked and nozzle 65a is released, the

pressure differential across the valve member 68 shifts this member to the left (FIG. 2), to increase the hydraulic fluid flow to line 69b.

Hydraulic fluid is delivered, under the control of the servovalve 62, to the hydraulic cylinder assembly 12 via a further valve 74 of the electromagnetically-operable type to return hydraulic fluid from line 75a to the reservoir 64 and to feed fluid under pressure to the line 75b. Lines 75a and 75b communicate with cylinder 12a of the drive means 12 on opposite sides of the piston 12b which is connected via a stem 76 with the electrode 10. The latter may be contoured to form, for example, a female die from the sintered particles 14.

Valve 74 has its solenoid 77 connected in series with a current source 78 and a limit switch 79 so that, upon actuation of switch 79 by a finger 80 carried by the stem 76 of the electrode 10, valve 74 cuts out the servocontrol valve 62 and applies full hydraulic pressure to the piston 12b. In this fashion the mass 14 may be compressed while -yet in a plastic state subsequent to discharge sintering under elevated pressure to further compact the body and increase the density thereof. The reed 71 is controlled by a pair of coils 84 and 85; coil 85 is connected across 'a source 86 via a variable-level rectifier 87 which delivers a reference potential to the voil 85. Coil 84 is energized by a servoamplifier 88 in circuit with the sensor 89 which, in turn, is connected across the electrodes 10 and 11.

The detector 89 produces a signal for energizing the amplifier 88 which is proportional to the current flow through the mass 14 between the electrodes 10 and 11, the voltage thereacross and/or thus to the power consumed in the sintering operation. In turn, the amplifier eriergizes control coil 84 of the reed 71 to adjust the position of the reed (and thereby apply hydraulic bias 'to the piston 12b) with respect to a reference input set at 87 to continuously reposition the electrode 10 to maintain the discharge conditions and controlledly follow the shrinking of the mass 14.

To control the degree of effect of the servomechanism, as described generally above, I provide means following the advance of the movable electrode for acting upon the control element of the servomechanism correspondingly. In the present case, the stem 76 of electrode 10 carries a cam 96 adapted to shift a cam follower 97 which, in turn, acts upon the effective fulcrum of reed 71. As the electrode .10 descends, therefore, the pressure delivered to the electrode 10 is augmented for an incremental change of the electrical parameters sensed by the servocontrol system by comparison with the normal response of the reed 71.

The cam 96 may have any desired configuration and may be designed to increase substantially the rate of advance of the electrode 10 and the pressure applied thereby to the sintering mass 14 during the terminal stages of the sintering operation. When finger 80 trips switch 79, valve 74 is shifted to inactivate the servocontrol valve 62 and supply hydraulic fluid at full pressure to the electrode for the subsequent compression of the mass.

To limit thermal variation of the electrical parameters to which the servomechanism responds, I provide the electrode 10, at its junction with stem 76, with a cooling coil 100 whose inlet 101 and outlet 102 may be fixed upon the stand 60 and connected with the coil 100 by flexible tubing 103, 104.

As has been noted earlier, it is advantageous to provide means for vibrating at least one of the electrodes during the initial development of spark discharge, especially when unidirectional power predominates as the discharge supply. In this case, the vibration may be carried out electromechanically as represented in FIG. 1 or electrohydraulically as illustrated in FIG. 2. The electrohydraulic means can include a coil 104a acting upon the reed 71 and energized by an A.C. source 104bof variable frequency and amplitude. As noted in connection with FIG. 1, this source may produce sonic or ultrasonic vibrations which are translated into corresponding oscillations of valve member 68 and, consequently, of the piston 12b.

It has also been found to be advantageous to employ, when the spark-sintered mass 14 is tungsten carbide or some other relatively hard wear-resistant material, a surrounding bed 15 of particles which are previously, subsequently or concurrently sintered into a support structure for the tungsten carbide die. Thus the particles 15 surrounding the mass may be composed of nickel, iron, aluminum, copper or the like and a spring-loaded annular electrode 10a, closely surrounding the principal electrode 10, may be provided. The electrode 10a is split into two arcuate segments across which a spark discharge is applied as represented by the leads 105, the power supply being of the type illustrated at 17 in FIG. 2. In this case, the bed 14 of tungsten carbide is bonded to the discharge-sintered mass 15 by a series of sparks propagated between the mass 15 and the particles 14, while the particles 15 are sintered together by discharge between the electrode segment 10a and the plate 11. Surrounding the mass 15 of particles having high thermal conductivity, I provide a further bed 15a of refractory particles (e.g. a mixture refractory oxides including, for example, calcium oxide, aluminum oxide, silicon dioxide and magnesium oxide).

In general, the system of FIG. 2 operates as follows: The powdered mass 14, consisting at least 80% by volume of conductive particles and no more than 20% by volume of nonconductive particles (e.g. metal oxides or thermoplastics), is disposed within a bed 15 of metallic particles of a different type. Electrode 10 is then brought to bear against the particle mass relatively lightly (e.g. with a pressure between 0.1 and 5 kg./cm. so that the movable electrode 10 can follow the particle mass and maintain contact therewith even upon shrinkage of the mass. The

pulser 17a is then actuated and spark discharged generated across'the electrode 10, 11 with each discharge of capacitor 18 and triggering of the controlled rectifier, the space discharge between these electrodes producing conductive bridges between the adjacent particles or between the nearest-neighbor particles of conductive material when a mixture of conductive and nonconductive elements is employed. With the formation of such conductive bridges at one side of the mass, say in the region of the electrode 11, individual discharges appear to be propagated through the remainder of the mass from the fritted particles at the electrode 11 to the still unbonded particles on the other side of the mass. These discharges strip the oxide film from the particles and break down any diffusion-resistant skins at the boundary surface of most metallic particles. The impulsive energy associated with these discharges impel the particles into contact at the resulting virgin surfaces so that dilfusion takes place with the formation of the aforementioned conductive bridges. An alternating current having a frequency between substantially Hz. and 10 kHz. can be applied across the electrodes 10, 11, as described in the aforementioned applications.

Also as set forth in my earlier copending applications and the aforementioned patents, a high-frequency source having an adjustable range between substantially 0.1 and 100 megaHz. is inductively coupled via a transformer and a DC. blocking capacitor across the electrodes.

As indicated earlier, it is of advantage to maintain the original gap parameters at least until an initial fusion occurs throughout the mass and the discharge has been propagated therethrough. Manual means for controlling the electric power delivered to the electrodes have proved ineffective because of the rapid change in the resistivity of the mass 14. The current and voltage sensor, however, responds instantaneously to the increased current flow and any changes in potential across the mass 14 and operates reed 71 to advance the electrode 10 such as to reestablish these gap conditions. This re-establishment of the gap conditions may involve an advance of the electrode at a rate less than that of shrinkage of the mass whereby, at least until an initial complete fusion, spark discharges continue to be generated in the space between the electrodes. Thereafter, as determined by cam 96, the electrode 10 may be advanced at a higher rate than required for more following of the shrinkage to increase the compression in positive feedback and thereby pass a fusion current through the mass subsequent to electric discharge; this latter current is concentrated at the conductive bridges and rapidly bring these bridged regions to a softening point at which the augmented pressure of the electrode (see US. Patent No. 3,241,956) compresses the mass further to densify it. Switch 79 may be tripped at this point to provide the increased pressure.

In FIG. 3, I show another servocontrol arrangement in which the servosystem is responsive to the power applied to the mass 214 between the electrodes 210 and 211. In this embodiment, the movable electrode 210 is represented as being driven by a reversible DC. motor 212 and to be energized by a direct-current impulse source 217 as well as an alternating-current source 243. Here the servocontrol system includes a Hall-effect crystal 262 which is energized in two mutually perpendicular directions by an electric signal and a magnetic field to produce an output in .the third coordinate direction perpendicular to these fields. Thus, two terminals of the Hall-effect crystal 262 are connected via the leads 263 across the electrodes 210 and 211 so as to apply the voltage of the D.C./A.C. source across the crystal in the direction of arrow 264 (along the y coordinate). A magnetic field is applied in the direction of arrow 265 (along the z coordinate) perpendicular to the plane of the Hall-effect crystal 262 by a yoke 266 whose coil 267 is energized through a current transformer 268 in series with the source 217, 243 and the electrodes 210, 211. The magnetic flux in the direction of arrow 265 can thus be represented at H where H =k l,k

being a constant and I representing the mean current flowing through the mass 214 of particles to be sintered. In accordance with the Hall-effect principle, the voltage E obtained in the direction of arrow 266, i.e. the potential gradient in the crystal plane perpendicular to the applied electromotive force V across terminals 263 and the magnetic flux H can be tapped across terminals 269. E the error signal for the servocontrol mechanism, can be represented as E=R -j H where j is the current density in the z direction of the applied voltage V and R is a constant. Since j- V, the latter equation can be rewritten as E,;=k V -H and, since Hwl, E=k -VI for the servocontrol output. The signal E is delivered to an amplifier or relay circuit 270 which, in turn, operates the reversible D.C. motor 212 by conventional means. The Halleffect crystal thus is a product-forming device responsive to both the potential of the sintering system and the current therethrough this product being a measure of the power consumed in the sintering process. It will be understood that a Hall-effect crystal and circuit as illustrated in FIG.- 3 can be substituted for the servocontrols of FIGS. 1 and 2 in accordance with the principles of the present invention or other means for sensing the electrical parameters of the sintering system may be provided. Furthermore, the current transformer of FIG. 3 may, instead of generating the magnetic flux perpendicular to the plane of the Hall-efiect crystal, be connected in one of the orthogonal directions of the crystal lattice while the voltage input to the servocontrol is delivered to the coil of the yoke. Moreover, while the magnetostrictive vibrator of FIG. 1 has not been shown in conjunction with the movable electrodes 10 and 210 of FIGS. 2 and 3, it will be understood that a vibrator of this type is generally provided in the system in place of or in addition to the source of alternating current 43 or 243.

In FIG. 3, I have diagrammed the principle here involved, the graph showing the voltage applied across the sintering mass plotted as a function of the current respectively along the ordinate and the abscissa with the voltage curves in solid line and the power curve in dot-dash line. The servomechanism is capable of controlling the power so that it remains substantially constant within the range represented at R and along the portion L of the power curve.

EXAMPLE I Using the system of FIG. 2 and applying a D.C. potential of 9 volts at 760 amps. upon which a pulsating current of 2 kHz. of 600 amps. was superimposed, 2.2 grams of nickel powder was sintered into a die having a diameter of 15 mm. in a period of about three seconds. The in1t1al hydraulic pressure of less than kg./cm. was built up to 12 kg./cm. during the sintering process and at the conclusion of sintering, the terminal pressure was raised to 200 kg./cm. via the valve 74. In the absence of a servomechanism, ile. when the electrode was permitted to follow the shrinkage of the mass under its own weight,

'the energy consumption was about 36 kilojoules/gram (kj./ g.) when the specific gravity of the body was 65% of that of'the solid material (nickel). When the specific gravity was raised to 70, 75 and 80%, respectively, the

discharge energies corresponded to 37.5, 42.5 and 46 'kj./g. without the servosystem. When only a voltagecontrolled servomechanism (cf. FIG. 2) was used at 65,

70, 75 and 80% specific gravity, the discharge energy and a voltage as measured across the crystal of 3 volts was applied, the error signal or output E was about 30 mv. and easily served to control the system.

In FIG. 5, I show a modified discharge-sintering arrangement in which the power supply is generally of the type illustrated in FIG. 2. Thus the power supply 317 comprises a pulsating source formed by a battery 317a in series with a surge-suppression choke 317b and the discharging capacitor 3170. The capacitor 3170 discharges through a solid-state controlled rectifier 317 which, upon firing to create the sintering pulse, is quenched by the oscillations generated by and in quenching choke 317d. An A.C.-bypass condenser 317k in shunt with the battery 317g applies the surge or pulse to the electrodes 310 and 311. The D.C. component of the sintering current is supplied by the battery 317g in series with a choke 317i. The particulate mass 314 retained in sleeve 316, is sandwiched between electrodes 310 and 311 which are individually displaced by piston-and-cylinder assemblies 312a and 312b, Each assembly 312a and 31% is controlled by a respective servovalve 312a and 312d supplied with hydraulic fluid from a pump 363 via lines 381a and 381b; the fluid returns by way of lines 364a and 364b to the reservoir 364. The servovalves 312s and 312dare individually operated by the servocontrol 312e in accordance with the voltage developed across the voltagedividing resistor 312 which detects an electrical parameter of the electrode gap. This dual servo ensures uniformity of sintering throughout the length of the sintered mass.

FIG. 6 shows a system for the welding of two plates together. The plates 410, 411 may be of similar or dissimilar material, bondable to the intervening layer 414 of powder. This system has been found to be especially suitable for welding together bodies of difiicult weldability. The plates 410 and 411 are held in respective electrodes 410a and 411a which are displaced by pistonand-cylinder assemblies 412a and 412b controlled by respective servovalves 412a and 412d by the servo circuitry 412e, representing either of the control arrangements of FIGS. 2 and 5. Hydraulic fluid is supplied to the servovalves 412c and 412d by the pump 463 and is returned to the reservoir 464. Between each cylinder 412a, 412b and the hydraulic lines, I provide a respective reversing valve 412c, 412d which permits the electrodes to be withdrawn and release the plates. A pulse/ D.C. source 417, of the type illustrated in FIGS. 2 or 5 and triggered by the pulser 417e, is connected across the electrodes 410a and 41%. During the spark sintering of the layer 414 of particles, retained by sleeve 416, together, the layer is bonded to both plates.

EXAMPLE II Using the apparatus illustrated in FIG. 6, two beryllium plates, each having dimensions of 25 mm. x 50 mm. x 10 mm., were welded together by a layer (ca. 1 mm.) of lOO-mesh beryllium powder. The initial pressure applied was about 2.5 kg./cm. and the welding current was composed of D.C. upon which 950 Hz. AC. is superimposed; the average current was 800 a./cm. at 15 volts and spark generation is evidenced in the gap. The advance of the individual plates was servocontrolled at the original gap parameters for 2 seconds; the final pressure was 250 kg./cm. The weld tested at a bonding strength of about 3,700 kg./cm.

As illustrated in FIG. 5, the pulser 317e or 417e may be constituted as a unijunction-transistor oscillator transformer coupled to the silicon solid-state controlled rectifier. Such an arrangement afiords ready control of pulse frequency and width. The unijunction oscillator comprises a unijunction transistor 317 one emitter-base network of which includes at the output side of the oscillator an output transformer 317k whose secondary winding is connected in triggering relationship with the gate of the controlled rectifier 317 A relaxation network, including a variable resistor 3171 and a capacitor 317m, is connected across the battery 317n and to the unijunction transistor 317 In FIG. 7, I show another modification of this invention wherein a pair of hydraulic cylinders are provided for the sintering electrodes and the lower cylinder is servocontrolled whereas the upper cylinder is provided for applying a high terminal pressure. In this arrangement, the electrodes 510 and 511 retain between them the powdered mass 514 of sinterable particles in a sleeve 516. The electrodes 510 and 511 which are provided with terminals 510a and Ella for the power supply and servo connections, are connected by insulating couplings 510b and 511b with the respective rods 512a and 51212 of the pistons of piston-and-cylinder assembly 512a and 51%. The upper or high pressure cylinder 512a is supplied with hydraulic fluid from a control valve 5120, a pump 563a and a reservoir 564a. A timer 580 may be employed to actuate the valve 5120 upon the lapse of a predetermined time period subsequent to initiation of sintering although automatic control of the valve 5120 is desirable as will become apparent hereinafter. The lower or servo cylinder 512k is supplied with hydraulic fluid from a pump 563b and a reservoir 564b via an oscillating valve 504a energized by a variable-frequency alternating-current source 504b. As in the system of FIG. 2, therefore, electrohydraulic oscillations can be imparted to the lower electrode 511 and sonic or ultrasonic frequencies via hydraulic control. Finally, the mechanism 504a, 504b is equivalent to that illustrated at 104a and -4b in FIG. 2.

The servomechanism of the embodiment of FIG. 7 comprises a valve 567 of the type illustrated in FIG. 2 wherein a pivotable reed 571 controls the efllux of fluid from the nozzles 565a and 565k which are illustrated in diagrammatic form. In accordance with the pressure drop permitted at these nozzles by the position of the reed 571, greater or lesser pressure is sustained in the branches 566a and 566k communicating with the cylinder 512b, thereby permitting control of the position of the electrode 511 in accordance with the movement of the reed '571. The reed 571 is, moreover, controlled by a reference coil 585 in series with an adjustable source 587 while an AC. source 586 may be connected in series therewith to further produce vibration of the electrode 511 by superimposing a vibrating current upon the reference voltage of the reed or flapper 571. The input to the valve 567 from the detector is applied at the coil 584 and derives from an amplifier 588 which obtains an arrow signal from an impedance 512 connected in series with the constantvoltage power supply of the system. It will be understood that the potential drop picked up across the detector 512i is a measure of the current flowing between the electrodes 510 and 511 which vary with the degree of compaction and coherency of the powdered mass when a constant-voltage power supply is provided. The constantvoltage power supply may be of any conventional type and is represented as including an adjustable D.C. source 517g in series with a choke 517i and controlled by a voltage regulator 517p. A pulse source 517 of the type illustrated at 17, 172 (or 317, 3178 as described above) superimposes a pulsating field upon the unidirectional current. A comparator 588a may be provided to compare the output of amplifier 588 with a reference input 588b and derive a control signal for operating valve 512c when spark fusion has essentially been completed or at any instant related thereto to compress the mass 514 at the terminal high pressure. In this embodiment, the self-compaction or gravitational efiect and the servo advance of the electrode 511 combined to permit excellent regulation of the distribution of the discharge throughout the mass. During the initial state of the sintering, the compaction of the mass 514 is followed by the servosystem 512 567, 51% and, at the conclusion of spark fusion, a terminalhigh pressure is applied by cylinder 512a.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the invention.

I claim:

1. An apparatus for the electric shaping of a conductive workpiece, comprising:

a pair of spacedly juxtaposed electrodes adapted to receive said workpiece between them;

drive means for controlledly advancing at least one of said electrodes continuously toward the other of said electrodes during the formation of said workpiece;

circuit means including a source of gap breakdown pulses connected across said electrodes for applying an electric potential across said electrodes of a level sufficient to cause at least an initial spark discharge between them to shape said workpiece; and servocontrol means for operating said drive means,

said servocontrol means including:

a Hall-efiect crystal having three mutually orthogonal electromagnetically responsive planes, first means for applying an electromagnetic signal across said crystal in one of said planes proportional to the electric current passing through said electrodes and said workpiece, second means for applying another electromagnetic signal across said crystal in another of said planes proportional to the voltage applied across said electrodes and said workpiece, and third means connected across said crystal in the third of said planes for deriving a signal proportional to the power consumption between said electrodes and for continuously operating said drive means to displace said electrodes to follow the shaping of said workpieces.

2. An apparatus for the spark-discharge sintering of a mass of discrete electrically fusible elements, comprising:

a pair of spacedly juxtaposed electrodes adapted to receive said mass between them; drive means continuously operable for advancing one of said electrodes toward the other of said electrodes upon the shrinkage of the volume of the mass between said electrodes during spark sintering thereof; means for applying an electric potential across said electrodes of a level sufiicient to effect breakdown of the gap therebetween and to eifect a spark discharge between them through said mass; and

servocontrol means responsive to at least one parameter of the spark sintering of the mass for continuously controlling said drive means to regulate the position of said one of said electrodes to follow the shrinkage of said mass.

3. An apparatus as defined in claim 2 wherein said servocontrol means includes detecting means in circuit with said electrodes and responsive to the electric-current flow through said electrodes and said mass for controlling said drive means in response to said electric-current flow.

4. An apparatus as defined in claim 2 wherein said servocontrol means includes detecting means connected in circuit with said electrodes and responsive to the voltage applied thereacross for controlling said drive means in response to said voltage.

5. An apparatus as defined in claim 2 wherein said servocontrol means includes detecting means responsive simultaneously to the electric-current flow through said electrodes and the voltage applied thereacross for operating said drive means at a rate determined by electric power consumption during sintering of said mass.

6. An apparatus as defined in claim 5 wherein said detecting means includes a Hall-effect crystal having three mutually orthogonal electromagnetically responsive planes,

first means connected in circuit with said electrodes and responsive to the electric-current flow therebetween for applying a signal across said crystal in one of said planes proportional to said electric-current flow, second means connected in circuit with said electrodes for applying across said crystal in another of said planes a signal proportional to thevoltage across said electrodes, and

third means connected across said crystal in the third of said planes for deriving from said crystal an output signal proportional to the power consumption in said mass and for controlling said drive means in response to said output signal.

7. An apparatus as defined in claim 2 wherein said drive means includes a reversible electric motor connected with said servocontrol means.

8. An apparatus as defined in claim 2, further comprising vibrating means connected with at least one of said electrodes for reciprocating same in the direction of the other of said electrodes at a frequency facilitating the de velopment of spark discharge between said electrodes.

9. An apparatus as defined in claim 2, further comprising means for inactivating said servocontrol means and applying an elevated pressure to said mass upon the spark sintering thereof to a predetermined extent.

10. An apparatus as defined in claim 2 wherein said drive means includes a respective piston-and-cylinder arrangement acting upon each of said electrodes for independently displacing same toward one another, said servocontrol means being operatively connected to one of said piston-and-cylinder arrangements for controlling same to maintain at least one electrical parameter of the spark sintering of the mass constant during at least an initial period of the sintering thereof, said apparatus further comprising means including the other of said piston-and-cylinder arrangements for applying an elevated terminal pressure to said mass subsequently to the initial period of sintering thereof.

11. An apparatus as defined in claim 2 wherein at least one of said electrodes is spark-sintered to said mass during the sintering thereof into a coherent state.

12. An apparatus for the spark-discharge sintering 'of a mass of discrete electrically fusible elements, comprismg:

a pair of spacedly juxtaposed electrodes adapted toreceive said mass between them;

drive means for advancing one of said electrodes to ward the other of said electrodes upon the shrinkage of the volume of the mass between said electrodes during spark sintering thereof;

electric-charge-storage means for applying an impulsive electric current across said electrodes to effect a spark discharge between them through said mass; and servocontrol means responsive to at least one parameter of the spark sintering of the mass for controlling said drive means, said drive means including fluid-responsive means for advancing said one of said electrodes, and a servocontrol valve having an electromagnetically operable member responsive to said parameter for controlling said fluid-responsive means,

13. An apparatus for the spark-discharge sintering of a mass of discrete electrically fusible elements, comprismg:

a pair of spacedly juxtaposed electrodes adapted to receive said mass between them;

drive means for advancing one of said electrodes toward the other of said electrodes upon the shrinkage of the volume of the mass between said electrodes during spark sintering thereof;

electric-charge-storage means for applying an impulsive electric current across said electrodes to effect a spark discharge between them through said mass; servocontrol means responsive to at leastone parameter of the spark sintering of the mass for controlling said drive means; and

a high-frequency alternating current generator connected across said electrodes for superimposing upon the gap-breakdown potential applied thereacross an alternating current of a frequency and intensity sufficient to promote the formation of spark discharge between said electrodes at a level of said potential below that at which gap breakdown would occur in the absence of said alternating current.

14. An apparatus as defined in claim 13, further comprising means for controlling the power ratio of said alternating current to the current of said charge-storage means.

References Cited UNITED STATES PATENTS 2,097,502 11/1937 Southgate 219-149 2,384,215 9/1945 Toulmin 2l9-149 3,240,961 3/1966 Noth 219 3,241,956 3/1966 Inoue 219-l49 JOSEPH V. TRUHE, Primary Examiner L. A. ROUSE, Assistant Examiner 

